The setting of Tech Romancer takes place in a far future of Earth, where advanced technology have made things calm and decent for the citizens of Japan and the rest of the world. However, the peace doesn't last long as an evil alien tyrant named Goldibus invades the planet with its loyal followers and seeks to conquer the world while enslaving the human race with an emotionless iron fist. An unlikely group of heroes band together to fight against the threat of Goldibus with their own unique mecha robots and all of them won't rest until Goldibus is defeated and the world is safe from the imminent danger.

Each mecha has its own story mode, which plays out like an anime series, with each battle broken up by an episode title, eyecatch, and dialog scenes before and after each battle. Each mecha has its own story (where it is the star of its own show), and decisions made in the dialog scenes, as well as the conditions under which a battle is won, can cause some stories to branch out and have multiple paths and endings. The other mecha and characters naturally make appearances, but their role may vary from their actual origins to fit the "star" mecha's story.

An "Arcade-style" mode where the player fights through each of the major mecha and bosses. Various hidden mecha and pilots found in the game can only be used in Hero Challenge Mode. In the Dreamcast version, points earned in Hero Challenge Mode and the VMU minigames could be used to purchase hidden characters including boss characters and movies.

Battles take place mostly on a flat 3D plane, with buildings and other terrain features scattered around. Destroying the terrain (by attacking or walking through them) releases power-ups, which include three weapons (vary between each character/mecha), armor or life powerups, and the Hero Mode powerup, which increases the power of your mecha's attacks, and may also unlock additional abilities or moves.

Rather than rounds, the matches are decided by the life meters of the fighters. Each fighter has two life meters, and is destroyed when the second one is depleted. In addition, each mecha gets an armor gauge that, when broken by consistent brute attacks, lowers the mecha's defense and makes it harder to recover from attacks received.

Each mecha has at least two super attacks, as well as a Final Attack, which is usable when the opponent is down to the last 50% of their second lifebar. This attack, when activated and successfully connected, automatically destroys the opponent, winning the battle.

Armor Knight G (Grand) Kaiser - A Super Robot similar to Mazinger Z, Grendizer, and Mazinkaiser. It is known as Kikaioh in the Japanese version, hence the title. G. Kaiser is the main protagonist robot of the game and its main story is like a typical Super Robot series (most notably Mazinger Z). This mecha attacks with rocket punches, bombs, and a heat beam in its chest. The pilot of G. Kaiser is Junpei Todoroki (voiced by Kōichi Tōchika): a hot-blooded main protagonist fighter (much like Mazinger Z 's main protagonist, Koji Kabuto) who leads the resistance against the invaders and seeks to defeat Goldibus in order to avenge his family's death (Junpei doesn't know that Shadow Red: a general in Goldibus' army is actually his missing brainwashed father). Although he's reckless and prone to tempered outbursts when provoked, Junpei is a good person who seeks to protect those that he cares about. G. Kaiser makes an appearance in the PTX-40A's ending in Tatsunoko vs. Capcom: Ultimate All Stars. It also appears in Project X Zone, as one of the mecha Devilotte (from Cyberbots) summons for her attack.

Super Defense Armour Dixen - A Mecha similar to the RX-78-2 Gundam as well as the Nu Gundam from the Mobile Suit Gundam series. It's armed with a shoulder-mounted beam cannon, grenades, and three remote-controlled beam guns. It can also switch between a beam rifle and a bazooka. The pilots of this mecha are Nakato Farland (voiced by Shigeru Nakahara): a calm and quiet man who finds himself being a reluctant fighter and Halma Flockheart (voiced by Yoshino Takamori): a young woman who has low self-esteem in combat. In Nakato's storyline, he is an escapee from his father's army, since he hates his important father for having started the war. He is accepted into Captain Baits' military force, who is fighting against Nakato's former allies. In the storyline, Shadow Red works for Nakato's father and is tasked with bringing Nakato back to his father's side. The storyline's first branch concentrates more on the hostility Nakato feels for his father, and Shadow Red uses Gourai in the final battle as an attempt to punish Nakato for his betrayal. For the storyline's second branch, Nakato is forced to fight and kill Yale, who in the storyline was a friend of his in training school, and this upsets him greatly. Also, in the final battle with Shadow Red, the latter uses another Dixen instead of Gourai, and his goal is to end his rivalry with Nakato instead of punishing Nakato for betraying his father. For Halma's storyline (who has only one route from within her storyline), she is an academy trainee who is at odds with her instructor, who for some reason constantly makes her participate in extremely difficult battle simulations that are not covered in the textbook. It turns out that the instructor in question is preparing Halma to defend Earth against the enemy, the Auto Machine Corps, and Halma must leave the planet without saying good-bye to her friends (which depresses her) in order to be in time to destroy the "queen unit." Goldibus, in this storyline, plays the role of the "queen unit", and after Halma destroys it, she celebrates with her best friend Raedd, although she mourns the fact that she feels the remorse of destroying a race.

Final Shot - In Dixen's Final Attack, it grabs the opponent and throws them high into the air, then draws its beam rifle and shoots them in the back. Dixen's pose during the Final Shot is a direct reference to the final shot (aka "Last Shooting") taken by the RX-78-2 in Mobile Suit Gundam.

Pulsion - An Ultraman-like humanoid alien hero who transforms into a giant to fight evil. Its abilities include "Space Karate", energy manipulation, and the ability to increase its physical strength. The true identity of Pulsion is unknown, as the mecha is either played by the male alien Kai (voiced by Takehito Koyasu) or the female alien Kei (voiced by Miki Nagasawa). It's uncertain what the relationship is between Kai and Kei, but it has been determined that the two of them may know each other from the past and that they help one another out from time to time while working together in the E.D.F. (Earth Defense Forces). In Kai's storyline, Pulsion was originally a servant of Goldibus who had its memory erased in a fight with Diana-17, who appears as a goddess-like figure in this story. Kai has the choice of continuing to fight for justice, or returning to the evil side and assuming its true purpose. For Kei's storyline, she finds herself having to fight both Goldibus and an evil version of herself while trying to determine on whether or not the human race is able to clean up the planet from the constant pollution that plagues it. Pulsion's design is a combination of mostly Ultraseven and partly Evangelion (especially in Hero Mode).

Strategic Variable Fighter Rafaga - A transformable combat aircraft in homage to the Macross series. It was designed by the famous mecha anime creator and designer Shoji Kawamori (who designed similar mecha in Macross).

Like the VF-1 Valkyrie from Macross, it can transform from jet fighter (which resembles the Orguss II, a mecha from an OVA with the same name by Studio Nue) to GERWALK and/or Soldier (mecha) mode.

Its pilot Simon Harvard (voiced by Takehito Koyasu) has some similarities with several Macross characters. He is a skirt chaser and ladies' man (similar to Roy Focker), who falls in love with an alien pilot (like Maximilian Jenius). The alien pilot in question is Yale, the pilot of Quvearl, and the fate of Simon's relationship with her is determined by the story path the player takes. In the first path, while Yale reciprocates Simon's feelings, she turns out to be an enemy robot programmed by Goldibus to spy on the military base and lead the enemy to destroy it. Yale is sorry, but feels she has no choice since she cannot go against Goldibus' programming. This causes Simon to decide the resolve in destroying Goldibus, since doing so would make Yale free. He succeeds, and with Goldibus' death, Yale is free, but doesn't know what to do from now on. So Simon proposes to her, and they share a kiss.

In the second path, Simon considers his life boring, since day after day, the routine is always the same despite his military success. He compares Yale to the angel in the Wonder Picture Book, his favorite story which he believes may someday become real. In this second story path, the military detains Yale before Simon even discovers Yale's true nature, and unlike the robot she was on the first story path, Yale turns out to be a missing "part" of Goldibus (whom Goldibus expects to re-merge with) that ran away because she didn't want to be found, which would have resulted in the destruction of the Earth, and Simon along with it. Simon defeats Goldibus, but is disappointed when Yale is not immediately waiting for him. He wanted his life to be like the Wonder Picture Book, with himself as the hero and Yale as his "princess." It turns out, however, that a young girl with blond hair who looks exactly like Yale was discovered where Goldibus had fallen.

Phantasm Unit Twinzam V - Another Super Robot, this one similar to combining robots like Getter Robo, with elements from Voltes V and Combattler V. Made up from a pair of jet fighters, it can transform between two forms depending on which is dominant. Twinzam V is alternately piloted by 10-year-old Daichi Yumeno (voiced by Rica Matsumoto) and his older sister, 11-year-old Sora (voiced by Mayumi Iizuka). The storyline of this mecha references several anime series - easily more than any other character. Regardless of which child's storyline the player is playing, both children end up piloting the Twinzam V in shared dreams, but are later trapped in the dream realm by Arekshim, pilot of Gamda. They are then recruited by Junpei, pilot of G. Kaiser, in the fight against Goldibus, but what happens after this recruitment depends on which storyline the player is playing. If the player is playing Daichi's storyline, then Sora loudly declares that she doesn't care what happens to anyone in this dream; she's only concerned that if she and Daichi die in this dream, they might die in real life. This fear causes Sora to leave, leaving Daichi to wonder why she is being so selfish. He, however, resolves to fight to save everyone in this dream even if he has to do it by himself. Right before the player's next battle (which is the final battle with Goldibus), Sora abruptly changes her mind and rejoins Daichi, citing as her rationale that she can't abandon her stupid brother Daichi, which makes the Twinzam V whole once again. In the ending, it is revealed that the whole incident was Daichi's dream all along, and that Sora does not remember their shared dreams from before. Daichi promises never to forget Junpei and their world in his own accord and time. If the player is playing Sora's storyline, then Sora realizes that the whole thing is HER dream, and she takes advantage of the situation to control what happens in the dream. She even makes Goldibus go berserk on purpose so that she and Daichi have to fight him; her goal is to behave in an unruly fashion until their mother (who it can be inferred is dead or otherwise forever absent in real life) comes to scold her, which would reunite Sora with their mother. The plan partially works; Sora is allowed to participate in a scene of their mother scolding her for making Daichi jump off the porch, and then telling Sora that she knows Sora is doing her best, and to please look after Daichi. Sora wakes up, yells at Daichi for tattling on her about the porch, and then explains that she got to meet their mother in her dream, which Daichi complains is unfair.

The Messenger of Beauty and Justice Diana-17 - Diana-17 is a fighting mecha piloted by Reika Amamiya (voiced by Michiko Neya), a strong-willed student at a girls' academy who is initially the first person to know of Goldibus' plans for invasion in the midst of a war. She tries to gather the other heroes in order to resist the invasion. Diana-17's design seems to be heavily influenced by ARIEL, although the control system resembles Jumborg Ace. Reika herself shares her clothing style with Domon Kasshu of Mobile Fighter G-Gundam fame.

Heavy Armour Tank Wise Duck - A huge weapon-laden mecha that Studio Nue says is a homage to the HWR-00-Mk. II Destroid Monster of Macross. Wise Duck is piloted by an entire crew, consisting of Gonzalles (Commander) (voiced by Daisuke Gōri), Arvin (Combat) (voiced by Shigeru Nakahara), Ricky (Gunner) (Voiced by Kōichi Tōchika), Thomas (Pilot) (voiced by Takehito Koyasu), and Herald (Sonar/Communications) (voiced by Shūichi Ikeda). This mecha pays some homage to the harder real robots, like Dougram. The Wise Duck teams is also highly reminiscent off the Cyclops Team of Gundam 0080. In the storyline, Arvin is a rookie military officer who, it turns out, has moral objections to some of war's uglier aspects, such as the necessity of having to kill very young pilots and especially the idea that the army Arvin belongs to considers it acceptable to burn a village of innocent people just so their enemy cannot hide. Depending on the player's actions in one of the fights, there are two possible story paths from here. In the first story path, Gonzales, Arvin's commander, abruptly changes his mind and decides to rebel against those cruel orders, because they would result in the massacre of civilians, which would necessarily include a little girl to whom Arvin earlier gave chocolate and who smiled at him. When Goldibus invades the Earth and cites humanity's hatred and warmongering as his reason, Arvin uses the little girl's smile as an example of something on Earth worthy to protect, inspiring him and the rest of the Wise Duck crew to defeat Goldibus. In the ending, Arvin is then shown as an older man over ten years later, the lone survivor among his crew, visiting the rusted Wise Duck, having heard that the village girl married someone and became a mother, and reflecting that he and his crew won against the enemy as humans. In the second story path, the crew "wakes up" many years into the future, in which humans are dominated by super-intelligent apes who dug up robots from the past. After the last few fights, the player abruptly learns that Arvin is in fact dead, having dreamed the whole thing as he was dying. This is because Gonazales shot Arvin for supposedly running from the enemy, and Thomas expresses regret that Arvin and his morals were in the wrong place, hoping that Arvin will dream and "wake up" in a time when there is no war.

Magical Patched Robot Bolon - This patchwork robot, resembling the Boss Borot from Mazinger Z or the titular patchwork robot from Roujin Z, created out of the parts of several buildings, is the creation of Pollialm "Pollin" De Pollintein (voiced by Satomi Koorogi), a Magical Girl who appears to be only about eight years old - nearly seen as Minky Momo / Gigi, although she can age herself to be roughly 17. She has a crush on Junpei, and ends up fighting several of the female pilots in the game over him in her storyline. She is actually the daughter of Goldibus.

Depending on the story branch in Bolon's storyline, Goldibus is either Pollin's father or her mother (though Goldibus' gender is officially male from within the main canon storyline of the game).

In the North American version of the game, a scene occurs in Pollin's story where she and Kei argue over who loves Junpei more. In the original Japanese version, they argue over who has the larger breasts (against Pollin's adult form).

Mariou Shogun Gourai - A huge red, Samurai-style mecha, complete with a sharp sword. Its design is similar to Red Geist from The Brave Fighter of Legend Da-Garn. One of Goldibus' generals, Shadow Red (who has only one route from within his storyline), has an agenda of his own that isn't known by Goldibus itself. In reality, he is Mamoru Todoroki, the father of Junpei who was kidnapped by Goldibus and brainwashed into becoming his loyal servant. Shadow Red also plays a role similar to Char Aznable in Dixen (Natako)'s storyline, and is voiced by Shuichi Ikeda. In addition, the mecha is styled similarly to Char's custom Gelgoog.

In addition to Gourai, Shadow Red also pilot Dixen Experiment in Dixen's storyline and the red Wise Duck with antenna in Wise Duck's storyline. Both mech are more powerful than standard type [1].

Strategic Variable Fighter Rafaga (Amuritta Type) - This red fighter is piloted by Amuritta, Simon Harvard's long-suffering wingman. Amuritta is a much more of a professional soldier than her commanding officer, but also harbors a small crush on him. She closely resembles Claudia LaSalle, who was the girlfriend of Roy Focker in the Macross series.

Quvearl - An Octopus-like bio-construct controlled by Yale, a female alien (one of many, as it turns out) who ends up falling for Simon Harvard after he stops her in a battle (See also Milia Fallyna Jenius). Its remote bit attacks and design resemble the Elmeth from Mobile Suit Gundam.

Gamda - A massive stone golem that seems to be made out of several temples with a huge eye in the middle. It's controlled by Arekshim (voiced by Ryūsei Nakao), an alien ruler with an Egyptian theme. His design is also seemingly inspired by Kaijuu.

Goldibus - The Ultimate Evil who serves as both the main antagonist of the series and the final boss of the game. Goldibus has two forms, in which his second form is more powerful and that he also has two menacing special attacks, unlike his less ominous first form. His design is inspired by the Great General of Darkness from the Mazinger anime metaseries, combined with Mechanical Beasts from the same series. Voiced by Chikao Ōtsuka.

Dr. Todoroki - An elderly expert scientist who happens to be the grandfather of Junpei and the father of Mamoru. He and Mamoru had helped develop G. Kaiser together from within an underground factory in order to be used against Goldibus and his invading forces. He dies after introducing Junpei to G. Kaiser and enabling him to control and utilize it from within G. Kaiser's storyline.

Dr. Tatsumi - An expert scientist and friend to Junpei's father and grandfather. He takes over in G. Kaiser's development and helps Junpei and his friends fight Goldibus from within G. Kaiser's storyline. In Gourai's storyline, however, he is a traitor, who tried to kill Junpei's parents. Shadow Red swears to get his revenge on him, but he is killed by Arekshim first.

Saori - Dr. Tatsumi's daughter, and Junpei's would-be girlfriend. Suffers a lot of misunderstandings in Bolon's storyline. Has a choker with a heart on it, like another character of Go Nagai's, Cutie Honey.

General - Commander of the armed forces from within both Nakato and Halma's versions of Dixen's storyline.

Chueng-Meng - A crewman on Nakato's ship from within Nakato's version of Dixen's storyline. She's very close to Nakato and worries about him a lot.

Raeed - One of Halma's classmates from within Halma's version of Dixen's storyline. Although he's not as skilled a pilot as Halma, he still looks after her when she's in trouble.

Commander - The leading officer of the E.D.F. (Earth Defense Forces) from within both Kai and Kei's versions of Pulsion's storyline.

Mr. Shima - The assistant of the E.D.F. Commander and second-in-command of the Earth Defense Forces from within both Kai and Kei's versions of Pulsion's storyline.

Goro - A young Japanese boy who befriends Kai and Kei from within both Kai and Kei's versions of Pulsion's storyline.

Maki - A childhood friend and classmate of Reika who goes to the same girls' academy as her from within Diana-17's storyline.

Shizuka - A snobbish and dismissive classmate of Reika who goes to the same girls' academy as her from within Diana-17's storyline. She greatly resembles Karin Kanzuki, a character from the Street Fighter series.